In the thermoforming process a sheet of thermoplastic material is heated until it becomes soft and moldable, but not fluid. The heated sheet is held against a mold, whereupon a vacuum is drawn between the mold and the plastic sheet, drawing the sheet down onto the mold, and causing the thermoplastic sheet to conform to the mold's surface. Thermoforming molds are typically fabricated of aluminum and water cooled. Thus, when a hot sheet of plastic comes into contact with the mold surfaces, it is cooled and retains the shape of the mold. Upon removal from the mold, the thermoformed article is allowed to further cool to approximately room temperature. Then the article is trimmed to leave only the desired molded shape.
Thermoforming may be used to fabricate articles from thick gauge stock. For the purposes of this application, thick gauge stock is defined to be thermoplastic material in the range of 60 thousandths to four-tenths of an inch thick or more. Articles commonly fabricated from such thick gauge stock include pallets, truck bed liners, utility containers, playground slides and numerous other articles.
A typical thermoforming machine known as a four-station rotary machine has four stations equally spaced about a circle. A carriage is positioned over the stations and has four subframes. Each subframe has pneumatically controlled clamps which clamp a sheet of plastic to each subframe.
The process of forming a thermoformed plastic article starts with loading an extruded thermoplastic sheet of a selected thickness into a subframe on the carriage. This operation is accomplished at a load/unload station. Once the sheet is loaded, the machine (normally under automatic control) rotates the carriage ninety degrees, bringing the sheet of plastic into position over a first heater. The heater normally consists of either electrical or gas radiant heaters which are positioned below the clamped sheet of plastic. After the sheet has been partially heated, it is rotated to a second heating station, again ninety degrees along the circular path of the carriage. Here, the sheet is brought to the final forming temperature, whereupon the sheet is rotated and brought into alignment with the forming station.
At the forming station, a water-cooled mold is brought up into contact with the sheet, normally forming an air-tight seal about the edge of the sheet with the edges of the mold. Air is drawn through small holes drilled through the surface of the mold to evacuate air from between the sheet and the mold. Thus, atmospheric pressure forces the plastic sheet against the mold cavity. After being allowed to cool on the mold, the sheet is removed from the mold, normally by injecting air through the vacuum holes to force the molded article off the mold.
From the forming station, the subframe containing the molded article is rotated on the carriage back to the load/unload station where the molded article is removed from the subframe clamps and replaced by a fresh sheet of thermoplastic material. After removal from the rotary thermoforming machine, the plastic article is often rough-trimmed. The article is then clamped in a fixture for final cooling which takes ten to fifteen minutes, depending on the thickness of the article and ambient air temperatures. Then, the fully cooled article is trimmed to final size.
The capital cost of the thermoforming machine represents a significant contribution to the final cost of the article manufactured. Another significant contribution to the cost of the article is the cost of the water-cooled aluminum mold on which the article is fabricated. Thus, in the past, efforts to improve the efficiency and reduce the cost of production of thermoformed plastic articles have endeavored to increase the number of items manufactured in a given amount of time on the thermoforming machine.
In many instances, the limiting factor in machine speed is found to be the rate of cooling of a thermoformed article. Plastic is not a highly thermally conductive material, and even on a water-cooled aluminum mold, cools at a relatively slow rate. An additional problem is that running the water-cooled mold at too low a temperature can result in the fabrication of inferior articles, caused by the too-rapid chilling of the portions of the plastic sheet which touch the mold first. Thus, the mold temperature is normally a function of process control. It is set at the level necessary to fabricate a high quality article, and is not readily adjustable to improved through-put by accelerated cooling.
In pursuit of increased production of thermoformed articles on a thermoforming machine, processes have been developed for cooling the molded article on the mold by spraying the surface of the article opposite the mold surface with cooling vapors, such as liquid carbon dioxide and liquid nitrogen. While these liquids can achieve a rapid cooling, they do so at considerable additional cost, both due to the material used and the additional cost of handling large quantities of cryogenic liquids and the evolved suffocating gases.
Another approach has been to position fans or air conditioning units over the mold. To the extent that cooling is done while the article is still on the mold to prevent distortion, the cooling is done in parallel with the molding step. This requires that the article remain on the mold until the cooling is completed. As subsequent articles may not be formed while the articles in such a process are cooling on the mold, the overall reduction of molding time is limited by these methods.
Yet another approach to increasing cooling has been a cooling station spaced from the mold and on the thermoforming machine. Such cooling stations have had a multiplicity of fans spaced about the part to blow ambient air on the part. Such an approach is limited in the rate of cooling that is possible and is prone to part distortion
What is needed is a thermoforming machine which can achieve higher through-puts by use of on-machine cooling without substantial cost increase or product distortion.